Mass rate control system for paper stock refiners

ABSTRACT

From consistency and flow data a signal is provided which is representative of the kilowatt hours per ton required to process the fluid paper stock passing through a refiner. This signal is compared with a desired set point and any difference is utilized as the set point for a power control loop. The function of the power control loop is to adjust the spacing of a longitudinally movable beater element contained in the refiner with respect to its stationary beater element. In this way, the power required by the drive motor that rotates the longitudinally movable beater element is maintained substantially uniform. The above action involves feedforward and feedback techniques. Additional feedforward correction is made for deviations in a preselected property, such as freeness, average fiber length and the like, of the incoming stock.

United States Patent lnventors Appl. No. Filed Patented Assignee MASSRATE CONTROL SYSTEM FOR PAPER STOCK REFINERS 2,727,694 12/1955Helmick,.lr.etal 3,078,051 2/1963 Patterson Primary Examiner-GranvilleY. Custer, .I r. AttrneyDugger, Peterson, Johnson & Westman ABSTRACT:From consistency and flow data a signal is provided which isrepresentative of the kilowatt hours per ton required to process thefluid paper stock passing through a refiner. This signal is comparedwith a desired set point and 14Cl 'ms lDrawin Fi g g any difference isutilized as the set point for a power control [52] US. Cl 241/37, loopThe f ti f the power Conn-0| loop is to adjust the 241/36 spacing of alongitudinally movable beater element contained [51] lltl. Cl B026 7/14i h refiner i h respect t it t ti b t l t [50] Field of Search 241/37,63, this w the power required by the drive motor that rotates 64, 36;162/253 254 the longitudinally movable beater element is maintainedsubstantially uniform. The above action involves feedforward and [56]References and feedback techniques. Additional feedforward correction isUNITED STATES PATENTS made for deviations in a preselected property,such as free- 1,815,155 7/1931 Lewellen et al 241/37 ness, average fiberlength and the like, of the incoming stock.

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PATENTED SEP] 4 l97l DESIRED TON SET POINT e 19 3 R] A EM 5 2 M E a v; a8 T E 3 M 3 w E m 2 w .H wwu w n m w T- mmp U. h w h h z H WM U 0 W R onIP-rift H EC 6 VI .ll v 0. m n\ nn .ufl |uU H I- I E] U R R m W TM mw 2we L 0 mm; m wk 9. m. Z W .m mm J mm) J mm m k Mm 5r T c1 4 I: .v PM T Tz 2| M w 0 0 /6 0 m0 K m 8 5 z 8 :w. Pc C C 2 5 K X c r umwwm r 2 0 01.L H 5 5 00 T m b RPM n. 2 5 5 SE 5 EWEKGY/ B ONE 0/? Y MASS RATE CONTROLSYSTEM FOR PAPER STOCK REFINERS BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention relates generally to refiners forprocessing fluid paper stock, and pertains more particularly to acontrol system utilizing consistency and flow information derived fromthe fluid stock passing through the refiner, as well as operating datarelating to a selected property of the incoming stock.

2. Description of the Prior Art Reference is made to U.S. Pat. No.3,309,031, issued Mar. 14, I967 to Richard F. McMahon et al. forMATERIAL WORKING APPARATUS, which patent is owned by the presentassignee. The control system described and claimed in said McMahon etal. patent hasperformed exceedingly well. However, the system set forthin said McMahon et al. patent is only flow sensitive. In certainrespects, the alluded-to prior art system has been too precise in itsoperation and certain difficulties have arisen, particularly where thedelta T (AT) transmitter is subjected to considerable vibration.

SUMMARY OF THE INVENTION Oneof the primary objects of the instantinvention is to provide a control system for paper stock refiners thatwill be responsiveto both flow and consistency. From the flow andconsistency data, a calculation is made that provides the basis for anaccurate positioning of the axially shiftable beater element so that thepower requirements of the drive motor that rotates this beater elementwillbe kept relatively uniform.

Another object is to provide a system of the foregoing character thatutilizes feedforward and feedback control principles.

Anotherobject of the invention is to constantly adjust the systemtocompensate for variations in a particular stock property, such asfreeness or fiber length.

Quite briefly, the present invention takes a set point indicative of thedesired number of kilowatt hours per ton which should be applied to aton of fluid .stock flowing through a refiner and compares this setpoint with a signal representative of the calculated kilowatt hours perton that has been derived from the consistency and flow data taken fromthe fluid stock passing through the refiner. Any difference between thetwo signals is utilized to provide an adjusted set point for a powercontrol loop which causes the movable beater element of the refiner. tobe positioned so that the load imposed upon the drive motor that rotatesthe movable beaterelement is maintained substantially constant. However,where a particular stock property varies sufficiently, the systemcorrects for the variation by immediately modifying the local set pointfor the power control loop.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE exemplifying theinvention is for the most part in block form but the refining apparatusis shown diagrammatically.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring .now in detail to thedrawing, it should beexplained atthe outset that the circuitryconstituting the invention may be operated in either of two modes, eachof which will become manifest asthe description progresses, and thateach mode can be promptly updated for a change in stock property whichif allowed to continue would result in a worsening of the powerstabilization.

Before describing the control systernit will be well to refer to typicalrefining apparatus with which the invention may be effectively;associated. Accordingly, a refiner has been designated generally by thereference numeral 10. In the form depicted the refiner is of theso-called disc variety and inconical plug-type refiner, such refinergenerally being'known as a Jordan refiner, in which a rotatable conicalplug is moved axially with respect to a complemental shell in which itis contained.

As illustrated, the refiner 10 in the present situation has a driveshaft 16 connected with the rotary element 14 and by means of anelectric drive motor 18 the shaft 16 is rotated 56 as to produce thedesired rotation of the element 14. Since the present invention isregarded as an improvement over 'said McMahon et al. patent, referencemay be made to this patent for further explanation as far as theconstruction of the refiner 10 is concerned. In the McMahon et al.patent, a pneumatic motor is referred to as positioning the movablebeater element with respect to the stationary element; in the presentsituation, it is planned that an electric motor 20 will be utilized.Such motor may function in the form described in U.S. Pat. No. 1,933,814issued on Nov. 7, 1933 to Darcy E. Lewellen et al. for STOCK CONSISTENCYCONTROL. Although not of recent innovation, the said Lewellen et al.patent is pertinent because it deals with a Jordan refiner and verysuccinctly shows a motor for rotating a threaded shaft that is employedfor positioning the conical plug with respect to the encompassing shell.

As far as the present drawing is concerned, it will be discerned thatthe fluid stock enters the refiner 10 through an inlet conduit 22 and isdischarged through an outlet conduit 23. In contradistinction to theaforesaid McMahon et al. patent, no attempt is herein made to determinethe temperature differential between the stock entering the refiner 10through the conduit 22 and the temperature leaving the refiner via theconduit 23. Instead, a consistency transmitter 24 is associated with theconduit 22, although the consistency of the fluid stock could bedetermined after the refining process has been completed, then beingassociated with the conduit 23. A correction factor would be introducedin order to compensate for any consistency change occasioned by therefinement that has transpired. Also utilized in the present controlsystem is a flow transistor 26 that provides a signal indicative of therate of flow of the fluid stock.

From the information derived from the transmitters 24 and 26, acalculator 28 computes the tonnage rate, doing so on a tons-per-hourbasis by multiplying the signals from the trans mitters 24 and 26together and also by an appropriate constant which will result in theunits being on the basis'of tons per hour. Hence, the calculator 28provides a signal that is representative of the massstock rate and morespecifically tons per hour.

A second calculator 30 is utilized for computing a signal that isindicative of the difference between a representative kilowatt value,which will be referred to more in detail hereinafter, and the actualkilowatts required by the motor 18 when the refiner 10 is operatingunder no-load conditions, that is without stock flowing therethrough.What the calculator 30 does is to accept the output signal from thecalculator 28 and divides or computes a ratio between the representativekilowatt value minus the no-load kilowatt value and the tonsper-hourmeasure obtained from the calculator 28. More 1 second input terminals32a, 32b and an output terminal 32c,- A

set point indicative of the amount of energy that desirably should beapplied to a given ton of fiber stock (desired KWH per bonedry ton)contained in the fluid stock flow entering the refiner through theconduit 22 is compared with the calculated kilowatt-hours-per-ton signalderived from the calculator 30, these signals being applied via theterminals 32a and 32b respectively, and any difference appears as anerror signal on the output line 32c. The error signal fed over theoutput line 320 is delivered to a kilowatt-hours-per-ton controller 34that provides an adjusted set point for a power control loop now to bedescribed.

Hence, the signal provided by the controller 34 is utilized in a powercontrol loop, being impressed on a summing junction or comparatorcircuit 36 through an input terminal 36a. Additionally, the comparator36 has a second input terminal 36b and an output terminal 360. The powercontrol loop further includes a power transmitter 38 of the wattmetertype which is electrically connected to the drive motor 18 so as toprovide a signal at the input terminal 36b which is representative ofthe actual power being used by said motor so that it can be comparedwith the set point signal arriving via the input terminal 360, thispower signal being forwarded over a line 37 connecting between thetransmitter 38 and the terminal 36b. It is any difference between theselast-mentioned signals that is reflected as an error signal at theoutput terminal 36c. The terminal 36c is connected directly tocontroller 40 that forwards an appropriate signal reflective of theerror to a pulse duration power controller 42 having a deadbanddetermined by selected limits doing so through a summing junction orcomparator circuit 44 having input terminals 44a, 44b and an outputterminal 44c. The 44b is included in a feedforward loop yet to bedescribed. However, with the information given up to this point in mind,it will suffice to state that whenever the error signal from thecomparator 36 is within prescribed upper and lower limits, thecontroller 42 will take no corrective action.

More specifically, it will be noted that the controller 42 is connectedto the control motor that functions to position the rotatable refiningelement 14 with respect to the stationary element 12. However, when theerror signal delivered via the terminal 36c is above or below theselected limits, the controller 42 then produces a pulse signal having aduration or span proportional to the difference or error sensed betweenthe input signal applied via the points 36a and 36b. Of course, thesignal arriving via the terminal 44b may alter the situation, as willpresently be made clear.

Earlier herein it was indicated that the calculator 30 accepts a no-loadkilowatt signal over line 30b. Now that the power transmitter 38 hasbeen referred to it can be explained that a line 45 connects directly tothe power line 37 and extends to a first selector switch 46 that hasbeen embodied in the drawing solely for the purpose of enabling a visualunderstanding of how the input signals to the calculator 30 areprocessed. In this regard, the switch 46 comprises a movable contact arm46a engageable with either of two fixed contacts 46b or 46a. The contact46b is attached to the line 45 and the contact 460 is dead, being an offcontact. The arm 46a is attached to a third contact belonging to theswitch 46 and it is this last-mentioned contact that is connected to theinput line 30b of the calculator 30.

Thus, to introduce the no-load kilowatt signal into the calculator 30,where it is stored for use in computing the output signal or ratio ofthe difference between the actual kilowatts and the no-load kilowatts,the arm 46a is moved from its depicted position into engagement with thecontact 46b. To determine the no-load or ineffective portion of powerthat does not contribute to refining, the beater element 14 is backedaway from the stationary element 12 to a point where no refining isbeing realized. Since this is the mechanical no load state, the powertransmitter forwards a signal representing the electrical power used bythe motor 18.

From the foregoing, it is to be noted that both feedforward and feedbacktechniques are employed to reduce system maladjustments. In this regard,feedforward action is provided by the power-per-unit-mass loop that isresponsive to flow and consistency disturbances and feedback action bythe power control loop that is responsive to actual power changes.

Inasmuch as two modes or methods of operation are con templated whenpracticing the invention, attention is now directed to the presence of asecond switch 48 that will serve to physically indicate what transpires,the switch 48 having a movable arm 48a and fixed contacts 48b, 48c and48d. When the switch arm 48a bridges the contacts 48b and 48d, the setpoint provided by the controller 34 is not only applied to the inputterminal 36a of the comparator 36 but is also applied to the input line30c of the calculator 30. Hence, the calculated kilowatt signal that isutilized in computing the ratio signal to"? be fed to the comparator 32is derived directly from the controller 34. In this instance, the setpoint signal introduced by way of the input line 300 has the no-loadkilowatt signal, which is entered via the line 30b, subtractedtherefrom. It is, therefore, the difference between the calculatedkilowatt signal and the no-load kilowatt signal that is ratioed with thetons-perhour signal arriving from the calculator 28 to furnish the inputapplied to the terminal 32b of the comparator 32.

On the other hand, when the switch arm 48a bridges the contacts 480 and48d, the actual kilowatt signal existing at any given moment isforwarded from the power transmitter 38, which is the same signalimpressed on the input terminal 36b of the comparator 36, over the inputline 300 to the calculator 30. Hence, in the latter situation or secondmode, the kilowatt hours per ton or work per unit mass is the same as inthe earlier described arrangement except that instead of using the powerset point from the controller 34, a measured power value is utilized inthe calculation performed by the calculator 30. In actual practice, theswitch 46 would not normally be used inasmuch as one or the other of theoperational modes would be selected in advance and used rather thanswitching between the two. However, the switch 48 serves as a facilemeans, as does the switch 46 with respect to the no-load data, forillustrating the two ways in which the control system herein describedcan function. In actual practice the system would be programmed for onemode or the other and would not make use of switches as such.

Inasmuch as certain properties of the stock entering the refiner 10through the inlet conduit 22 will affect the operation of that portionof the system described up to this point, provision is made foreffecting a prompt change to take care of the variation in stockproperty that has occurred. Since the specific stock property orcharacteristic is unimportant, such property will simply be denoted asZ. The property might well be concerned with fiber dimensions, theirflexibility, their freedom of movement or any other characteristic thatshould not be allowed to deviate from an acceptable norm. Z,, willtherefore be employed to denote the measured stock property, a stockproperty transmitter forwarding an appropriate signal indicative of theproperty Z. The desired stock property will be denoted as Z,, and a setpoint signal representative thereof applied to a summing junction orcomparator 52. More specifically, the set point signal is supplied viaan input terminal 52a and the signal from the transmitter 50 via asecond input terminal 52b, any difference or error between the inputsignals appearing at the output terminal 52c of the comparator 52.

Since the purpose of the feedforward loop currently being described isto introduce into the overall system a corrective action which is afunction of the incoming stock property 2, it will be necessary toprovide a control signal that will possess the proper weight orinfluence. Consequently, a simple calculator 54 comprising a multiplier54a and adder 54b will visually portray what can be employed, althoughin actual practice all of the computations, that is those performed bythe calculators 28 and 34 as well, could be handled by a single computeror each with a suitable computer/controller. Hence, the differencesignal (Z, Z,,,) at the output terminal 52c might be multiplied by aconstant K, and the product added to a constant K to provide therequisite control. Although a conventional computer/controller could beemployed, as indicated above, a controller 56 has been shown, thecontroller 56 receiving the output from the calculator 54 and supplyingthe previously alluded-to input terminal 44b with an appropriate signal.Any predetermined difference between the signals arriving via the twoterminals 44a, 44 b will influence the action of the deadband controller42 that controls the beater adjusting motor 20.

In view of the detailed description that has been given, coupled signala great degree with the operational sequence that occurs, an elaboratedescription of the operation need not be presented. However, it shouldbe remembered that a set point is provided for the power control loop,this being the signal applied to the input terminal 36a, that is derivedfrom actual operational data involving both flow and consistency. Itwill be appreciated that the flow and consistency determinations aremade in a kilowatt-hours-per-ton control loop which includes thecalculators 28, 30, the comparator 32 and the controller 34. Anydifference between such a calculated set point signal and the actualpower signal from the transmitter 38 is utilized in the control of themotor 20, by way of the controller 40 and the deadbanded powercontroller 42; which action determines the spacing between the refiningelements 12 and 14. In this way, the power requirements of the motor 18are maintained at a substantially uniform and desired level, anyfluctuations or variations being rather minimal even where theconsistency of the fluid stock entering through the conduit 22 and itsrate of flow may vary quite widely.

However, when the measured stock property signal Z,, deviates or shiftssufficiently from the desired stock property signal Z,,, a relativelyrapid adjustment is made of the beater element 14 with respect to theelement 12 so that compensation is effected through the controller 56without the delay that would otherwise occur only through the feedbackcontroller 40. Therefore, it will be appreciated that a secondfeedforward loop is provided through the agency of the transmitter 50and the controller 56 associated therewith, this loop being in additionto the feedforward loop which includes the transmitter 24, 26' and thecontroller 34.

I claim:

1. In combination with a refiner for processing fluid paper stock whichincludes a pair of relatively rotatable and axially movable refiningelements, a drive motor for relatively rotating said elements and acontrol motor for axially shifting one of said elements with respect tothe other, a control system comprising a power control loop responsiveto the actual power consumed by said drive motor to operate said controlmotor in accordance with any difference between a signal representativeof the actual power consumed by said drive motor and a power set pointsignal, and an energy-per-unitmass control loop responsive to rate offlow and consistency data derived from the fluid stock passing throughsaid refiner to provide a calculated energy per unit mass signal whichis compared with a set point signal indicative of a desired energy perunit mass so as to provide said power set point signal in accordancewith any difference therebetween, whereby the power required by saiddrive motor is relatively uniform.

2. The combination of claim 1 in which said energy-perunit-mass controlloop includes first calculating means responsive to said rate of flowand consistency data for providing a signal representative of tons perhour of stock flowing through said refiner, second calculating means forproviding a signal that is the ratio of the difference between theno-load power value required by said refiner for the particular flowcondition and a power value with respect to the value of saidtons-perhour signal, and means for comparing the value of said ratiosignal with said set point signal that is indicative of the desiredenergy per unit mass to provide said power set point signal.

3. The combination of claim 2 in which the value of said power set pointsignal constitutes the power value, the no-load power value beingsubtracted from the value of said power set point signal by said secondcalculating means to provide said difference.

4. The combination of claim 2 in which the value of the actual powerconsumed by said drive motor constitutes the power value, the no-Loadpower value being subtracted from the value of the actual power by saidsecond calculating means to provide said difference.

5. The combination of claim 1 including a stock property loop responsiveto the value of a given stock property for modifying the amount ofactual power supplied to said drive motor in accordance with thedifference between the measured value of said given stock property andthe value of a desired value of said given stock property.

6. In combination with a refiner for processing fluid paper stock whichincludes a pair of relatively rotatable and axially movable refiningelements, a drive motor for relatively rotating said elements and acontrol motor for axially positioning one of said elements with respectto the other, a control system comprising a power control loop includinga power transmitter for providing a signal which is a measure of theactual power being consumed by said drive motor, comparator means forcomparing said actual power signal with a power set point signal toprovide an error signal representative of any difference therebetween,means responsive to said error signal for operating said control motorin accordance with the value of said error signal so as to position saidone refining element axially with respect to said other refiningelement, means providing a signal which is a measure of the flow rate offluid stock through said refiner, means providing a signal which is ameasure of the consistency of said fluid stock, first calculating meansfor computing a mass stock rate signal from said flow and consistencysignals, second calculating means for computing a signal representativeof energy per unit mass, said energy-per-unit-mass signal being derivedfrom a ratio of the difference between first and second signals withrespect to said mass stock rate signal wherein said second signal isrepresentative of the power required by said refiner under no-loadconditions, comparator means for comparing said energy per unit masssignal with a set point signal indicative of a desired energy per unitmass to provide an error signal in accordance with any differencetherebetween, and means responsive to said last-mentioned error signalfor providing said power set point signal,

7. The combination of claim 6 in which said first signal corresponds tosaid power set point signal.

8. The combination of claim 6 in which said first signal corresponds tosaid signal which is a measure of the actual power being consumed bysaid drive motor.

9. The combination of claim 6 including the means responsive to ameasured stock property for modifying the value of said first-mentionederror signal in accordance with any difference between said measuredstock property and the set point representing a desired stock property.

10. In combination with a refiner for processing fluid paper stock whichincludes a pair of relatively rotatable and axially movable refiningelements, an electric drive motor for relatively rotating said elementsand an electric control motor for axially positioning one of saidelements with respect to the other, a control system comprising a powercontrol loop including a power transmitter for providing a signal whichis a measure of the actual power being consumed by said drive motor,first comparator means for comparing said actual power signal with apower set point signal to provide a first error signal representative ofany difference therebetween, a deadbanded pulse duration powercontroller responsive to said error signal for furnishing pulses to saidcontrol motor having a duration proportional to the value of any sucherror signal so as to position said one refining element with respecttosaid other refining element in accordance with the length of pulsessupplied by said controller, means providing a signal which is a measureof the flow rate of fluid stock through said refiner, means providing asignal which is a measure of the consistency of said fluid stock, firstcalculating means for computing a signal representative of tons per hourof stock flowing through said refiner, second calculator means havingfirst, second and third input lines, said first input line beingconnected to said first calculating means, said second input lineintroducing into said second calculating means a given kilowatt signaland said third input line introducing into said second calculating meansa signal representative of the kilowatts required by said refiner underno-load conditions, said second calculating means ratioing thedifference between said signals introduced via said second and thirdinput lines with respect to the signal introduced via said first inputline to provide an output signal representative of calculated kilowatthour per ton, second comparator means for comparing said calculatedkilowatt-hours-per-ton signal with a set point signal representative ofa desirednumber of kilowatt hours per ton to provide a second errorsignal in accordance with any difference therebetween, and akilowatt-hours-per-ton controller responsive to said last-mentionederror signal for providing said power set point signal.

11. The combination of claim 10 in which said power set point signalconstitutes said given kilowatt signal introduced via said second inputline to said second calculating means.

12. The combination of claim 10 in which said actual power signalconstitutes said given kilowatt signal introduced into said secondcalculating means via said second input line.

13. The combination of claim 10 including a third comparator meansdisposed between said first comparator means and said deadbanded pulseduration power controller for comparing said first error signal with astock property signal having a value representative of any change thatsaid stock property signal has deviated from a desired value so as toprovide a third error signal in accordance with the difference betweensaid stock property signal and said first error signal, said third errorsignal controlling said deadbanded pulse duration power controller inaccordance with the value thereof.

14. The combination of claim 13 including a stock property transmitterfor providing a signal which is the measure of the stock property ofstock flowing through said refiner, and a fourth comparator means forcomparing said measured stock property signal with a set point signalrepresentative of the stock property that is desired so as to producesaid third error signal.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,604,646 Dated September 14, 1971 Inventor) Marion A. Keyes, IV andJohn A. Gudaz It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 42, "transistor" should be --transmitter--. .1 Column 5,line 9, after "pled" delete "signal" and insert -to--.

I Signed and sealed this 29th day of February 1972.

(SEAL) Atte'st:

EDWARD M.FLETCHER,JH. ROBERT GOT TSCHALK Attesting Officer Commissionerof Patents

1. In combination with a refiner for processing fluid paper stock whichincludes a pair of relatively rotatable and axially movable refiningelements, a drive motor for relatively rotating said elements and acontrol motor for axially shifting one of said elements with respect tothe other, a control system comprising a power control loop responsiveto the actual power consumed by said drive motor to operate said controlmotor in accordance with any difference between a signal representativeof the actual power consumed by said drive motor and a power set pointsignal, and an energy-per-unit-mass control loop responsive to rate offlow and consistency data derived from the fluid stock passing throughsaid refiner to provide a calculated energy per unit mass signal whichis compared with a set point signal indicative of a desired energy perunit mass so as to provide said power set point signal in accordancewith any difference therebetween, whereby the power required by saiddrive motor is relatively uniform.
 2. The combination of claim 1 inwhich said energy-per-unit-mass control loop includes first calculatingmeans responsive to said rate of flow and consistency data for providinga signal representative of tons per hour of stock flowing through saidrefiner, second calculating means for providing a signal that is theratio of the difference between the no-load power value required by saidrefiner for the particular flow condition and a power value with respectto the value of said tons-per-hour signal, and means for comparing thevalue of said ratio signal with said set point signal that is indicativeof the desired energy per unit mass to provide said power set pointsignal.
 3. The combination of claim 2 in which the value of said powerset point signal constitutes the power value, the no-load power valuebeing subtracted from the value of said power set point signal by saidsecond calculating means to provide said difference.
 4. The combinationof claim 2 in which the value of the actual power consumed by said drivemotor constitutes the power value, the no-Load power value beingsubtracted from the value of the actual power by said second calculatingmeans to provide said difference.
 5. The combination of claim 1including a stock property loop responsive to the value of a given stockproperty for modifying the amount of actual power supplied to said drivemotor in accordance with the difference between the measured value ofsaid given stock property and the value of a desired value of said givenstock property.
 6. In combination with a refiner for processing fluidpaper stock which includes a pair of relatively rotatable and axiallymovable refining elements, a drive motor for relatively rotating saidelements and a control motor for axially positioning one of saidelements with respect to the other, a control system comprising a powercontrol loop including a power transmitter for providing a signal whichis a measure of the actual power being consumed by said drive motor,comparator means for comparing said actual power signal with a power setpoint signal to provide an error signal representative of any differencetherebetween, means responsive to said error signal for operating saidcontrol motor in accordance with the value of said error signal so as toposition said one refining element axially witH respect to said otherrefining element, means providing a signal which is a measure of theflow rate of fluid stock through said refiner, means providing a signalwhich is a measure of the consistency of said fluid stock, firstcalculating means for computing a mass stock rate signal from said flowand consistency signals, second calculating means for computing a signalrepresentative of energy per unit mass, said energy-per-unit-mass signalbeing derived from a ratio of the difference between first and secondsignals with respect to said mass stock rate signal wherein said secondsignal is representative of the power required by said refiner underno-load conditions, comparator means for comparing said energy per unitmass signal with a set point signal indicative of a desired energy perunit mass to provide an error signal in accordance with any differencetherebetween, and means responsive to said last-mentioned error signalfor providing said power set point signal.
 7. The combination of claim 6in which said first signal corresponds to said power set point signal.8. The combination of claim 6 in which said first signal corresponds tosaid signal which is a measure of the actual power being consumed bysaid drive motor.
 9. The combination of claim 6 including the meansresponsive to a measured stock property for modifying the value of saidfirst-mentioned error signal in accordance with any difference betweensaid measured stock property and the set point representing a desiredstock property.
 10. In combination with a refiner for processing fluidpaper stock which includes a pair of relatively rotatable and axiallymovable refining elements, an electric drive motor for relativelyrotating said elements and an electric control motor for axiallypositioning one of said elements with respect to the other, a controlsystem comprising a power control loop including a power transmitter forproviding a signal which is a measure of the actual power being consumedby said drive motor, first comparator means for comparing said actualpower signal with a power set point signal to provide a first errorsignal representative of any difference therebetween, a deadbanded pulseduration power controller responsive to said error signal for furnishingpulses to said control motor having a duration proportional to the valueof any such error signal so as to position said one refining elementwith respect to said other refining element in accordance with thelength of pulses supplied by said controller, means providing a signalwhich is a measure of the flow rate of fluid stock through said refiner,means providing a signal which is a measure of the consistency of saidfluid stock, first calculating means for computing a signalrepresentative of tons per hour of stock flowing through said refiner,second calculator means having first, second and third input lines, saidfirst input line being connected to said first calculating means, saidsecond input line introducing into said second calculating means a givenkilowatt signal and said third input line introducing into said secondcalculating means a signal representative of the kilowatts required bysaid refiner under no-load conditions, said second calculating meansratioing the difference between said signals introduced via said secondand third input lines with respect to the signal introduced via saidfirst input line to provide an output signal representative ofcalculated kilowatt hour per ton, second comparator means for comparingsaid calculated kilowatt-hours-per-ton signal with a set point signalrepresentative of a desired number of kilowatt hours per ton to providea second error signal in accordance with any difference therebetween,and a kilowatt-hours-per-ton controller responsive to saidlast-mentioned error signal for providing said power set point signal.11. The combination of claim 10 in which said power set point signalconstitutes said given kilowatt signal introduced via said second inputline to said secoNd calculating means.
 12. The combination of claim 10in which said actual power signal constitutes said given kilowatt signalintroduced into said second calculating means via said second inputline.
 13. The combination of claim 10 including a third comparator meansdisposed between said first comparator means and said deadbanded pulseduration power controller for comparing said first error signal with astock property signal having a value representative of any change thatsaid stock property signal has deviated from a desired value so as toprovide a third error signal in accordance with the difference betweensaid stock property signal and said first error signal, said third errorsignal controlling said deadbanded pulse duration power controller inaccordance with the value thereof.
 14. The combination of claim 13including a stock property transmitter for providing a signal which isthe measure of the stock property of stock flowing through said refiner,and a fourth comparator means for comparing said measured stock propertysignal with a set point signal representative of the stock property thatis desired so as to produce said third error signal.